1. Field of the Invention
This invention relates to a pneumatic tire having a rolling resistance reduced by decreasing strain energy loss in a sidewall portion of the tire.
2. Description of the Related Art
In general, when the pneumatic tire is run, the sidewall portion is repeatedly deformed under a load at a ground contact side, during which strain energy such as bending energy, shearing energy and the like are stored in the rubber constituting the sidewall portion (hereinafter referred to as sidewall rubber) by the above deformation. Thereafter, when the sidewall rubber is released from the deformation by shifting from the ground contact side, the aforementioned strain energy is converted into heat energy to bring about useless energy consumption. For this end, the pneumatic tire usually has a large rolling resistance.
Heretofore, there have been various pneumatic tires having a rolling resistance reduced by decreasing strain energy loss in the sidewall portion. For example, it is known that the sidewall rubber is divided into outer rubber layer and inner rubber layer and rubber having a low modulus is used as the inner rubber layer, whereby strain energy loss in the inner rubber layer is reduced to reduce strain energy loss of the sidewall portion as a whole.
In such a conventional pneumatic tire, however, the sidewall portion is rendered into a two-layer structure, so that the tire build-up step becomes complicated and hence the productivity is lowered and the cost of the tire product becomes high.